Recently, multilayer structure has been widely employed in printed wiring boards for miniaturization of the printed wiring boards by increased wiring density of printed wiring boards. Multilayer printed wiring boards are used in many mobile electronic devices to achieve downsizing. Then, multilayer printed wiring boards are required both further thickness reduction in interlayer insulating layers and further weight reduction.
As a technology which satisfies such requirements, a manufacturing method employing the coreless build-up method disclosed in Patent Document 1 has been used. The coreless build-up method is a manufacturing method for forming a multilayer structure without using the so-called core substrate, i.e. the method alternately laminate layers of an insulating layer constituting material consists of only a polymer material and a conductor layer. In the market, utilization of a copper foil with a peelable-type carrier in the coreless build-up method has been introduced to make separation between the multilayer printed wiring board and the supporting substrate easy. Then, the following products are available as the copper foil with carrier used in the coreless build-up method.
For example, the copper foil with carrier disclosed in Patent Document 2 is used in the coreless build-up method. In Patent Document 2, an ultra-thin copper foil with carrier having a layer structure consists of “a carrier foil, a diffusion prevention layer, a release layer and an ultra-thin copper foil” is used to achieve the objects; to provide an ultra-thin copper foil with carrier without blister in the releasing interface, low carrier peel, friendly to the environment and easy carrier foil releasing from the ultra-thin copper foil after keeping in a high temperature environment; and to provide a printed wiring board backed up by stable manufacturing quality as a base material for a printed wiring board used in fine pattern application using the ultra-thin copper foil with carrier. In the ultra-thin copper foil with carrier, the release layer consists of a metal A for ensurance of releasability and a metal B for easy plating of the ultra-thin copper foil, wherein the release layer consists of the metal components in which the content [a] of the metal A and the content [b] of the metal B in the release layer satisfy the ratio [a]/[a+b]*100 of 10 to 70%.
In the manufacturing of a printed wiring board employing the coreless build-up method disclosed in Patent Document 1, a printed wiring board can be manufactured by employing the coreless build-up method using the ultra-thin copper foil with carrier disclosed in Patent Document 2. In this method, the manufacturing process of the printed wiring board employing the coreless build-up method is shown in FIG. 4 to FIG. 6. As is apparent in the process, after the build-up layer to be an actual board later is formed on the supporting substrate, the supporting substrate and the build-up layer are separated at the release layer in the ultra-thin copper foil with a peelable-type carrier provided between the supporting substrate and the build-up layer. In this step, in the ultra-thin copper foil with carrier, the diffusion prevention layer (heat-resistant metal layer) which prevents diffusion of the release layer components exists on the carrier side to ensure the separation at the release layer even after loading a high temperature in lamination.